The invention relates to a process for the preparation of a starch ester of a number average molecular weight of 1000-50,000 and a degree of substitution of 0.4-3.0 by esterification of a starch with one or more monocarboxylic acids. By starch is to be understood here a starch which may or may not be degraded and/or alkoxylated. By degree of substitution is to be understood here the number of substituents, up to a maximum of 3.0 anhydroglucose unit.
In actual practice, the preparation of starch esters having a low degree of substitution is usually carried out by re-esterification with a vinyl ester or by reaction with acetic anhydride under alkaline or neutral conditions. The preparation of a starch ester having a high degree of substitution generally takes place in an organic solvent, use being made of acetic anhydride or an acid chloride. The large scale preparation in this way of starch ester from a higher monocarboxylic acid, however, is unattractive because of the lack of commercial anhydrides or acid chlorides. A problem to the preparation of starch esters is the poor solubility of starch in usual organic solvents and, moreover, starch is poorly accessible to organic compounds, which generally results in a low reactivity.